1. Field of the Invention
This invention is related to an automatic transmission control system for automobiles, and, in particular, related to an automatic transmission control system in which utilization is made of pressure regulating valves to regulate the coupling pressure supplied to frictional coupling elements.
2. Description of Related Art
In automatic transmissions for automobiles, a transmission gear mechanism is coupled to a torque converter, and a drive torque transmission path of the transmission gear mechanism is changeable through the selective coupling of a plurality of frictional coupling elements so as to automatically shift the automatic transmission to various gears according to vehicle driving conditions. In such an automatic transmission, a hydraulic controlling circuit controls coupling pressure supplied to the frictional coupling elements. Japanese Unexamined Patent Publication No. 1-150055 discloses a hydraulic control circuit of this type, which has a linear solenoid valve to control coupling pressure supplied to each of the frictional coupling elements. This hydraulic control circuit includes a regulating valve disposed in a pressure line to the frictional coupling elements. The control pressure supplied to the regulating valve is developed by means of the linear solenoid valve. In this instance, a control signal is output to the linear solenoid valve from a controller so as to develop the control pressure supplied to the regulating valve, thereby appropriately controlling the coupling pressure applied to the frictional coupling elements. For example, through the adjustment of torque transmission capacity of the frictional coupling element relative to an input torque to the frictional coupling element, the frictional coupling element reliably transmits torque without loss in driving force of an oil pump attributable to an unnecessary increase in torque transmission capacity.
With this type of hydraulic control circuit, when supplying coupling pressure to the frictional coupling element during gear shifting, in order to reduce "shift shock," the coupling pressure to the frictional coupling element is regulated to an interim level of pressure, otherwise known as a "rack pressure," during the gear shifting. However, it is intended to control or regulate the coupling pressure without making it too little or in excess to have quite a broad range of coupling pressure control. If in fact the regulation valve is required to have such a broad range of pressure control, as is particularly characteristic of the linear solenoid valve, precise control becomes difficult, resulting in a reduction in pressure control accuracy.